The present invention is based upon several technologies, namely chemical sensors and biosensors, chemical modification of surfaces, and interferometer technology.
A major goal in sensor technology is toward smaller size and greater sensitivity. Also, increasingly stricter demands of monitoring specificity and selectivity are being imposed, particularly in fields such as environmental monitoring and control, physiological monitoring, diagnostic monitoring, bioprocessing, agriculture, pharmaceuticals, therapeutic monitoring, and even defense and petrochemicals.
A "sensor" is a type of transducer; i.e., a device that responds to an external stimulus or input signal by producing a measurable response having a magnitude bearing a relationship to the magnitude of the external stimulus or input signal.
A "chemical sensor" is a sensor in which a chemical reaction or molecular change in or on the sensor is an important aspect of the production of a measurable response by the sensor.
A "biosensor" is a sensor that incorporates a biological or biomolecular component as a key functional element in the production of a measurable response by the sensor.
Biosensors have been the subjects of great attention due to the high specificity of interactions of many types of biomolecules with molecules of other compounds. See, e.g., Vadgama et al., "Biosensors: Recent Trends, "Analyst 117:1657-1670 (1992). However, bridging the gap between knowledge of a particular reaction involving biomolecules and the exploitation of the reaction in a biosensor has often proven to be difficult. For example, in biosensors the biological component is usually in the form of a biolayer that is frequently metastable. Thus, many contemporary biosensors are subject to obfuscating environmental influences. Secondly, biolayers usually need to directly contact the analyte which is frequently present in a complex mixture comprising a large number of other compounds that can interfere with the response of the biolayer to a target analyte or that can be interfacially active and/or possibly detrimental to the biolayer.
An important problem often encountered in making sensors, particularly biosensors, is how to immobilize molecules of the sensing compound (i.e., the "sensing molecules") to a particular situs in or on the sensor such as an appropriate substrate surface. Such immobilization cannot substantially adversely affect the ability of the sensing molecules to respond to significant changes in the measured parameter when the sensing molecules are exposed or otherwise contacted with molecules of an analyte. Such immobilization requires that the sensing molecules retain their reactive specificity toward the corresponding analyte even when the sensing molecules are attached to the situs.
One way to immobilize sensing molecules to a situs is to chemically bond them to the situs. However, particularly with biomolecules, immobilization of sensing molecules by conventional bonding techniques can cause the sensing molecules to change conformation or undergo any of several other changes that can reduce or destroy the capacity of the sensing molecules to respond to the analyte.
Another problem often encountered is that, whereas many substrates such as polymeric substrates have properties that render them desirable for use as substrates, it is often difficult or impossible by contemporary methods to bond sensing molecules to them, particularly using chemistry that does not cause damage to the substrate, the sensing molecules, or both.
Attaching sensing molecules to a substrate can be thought of as a form of chemical modification of, or "functionalization" of, a substrate.
Chemical modification of surfaces has been the subject of intensive research. Examples of such surfaces include polymers, Braybrook et al., Prog. Polym. Sci. 15:715-734 (1990); metals, Stratmann, Adv. Mater. 2:191-195 (1990); silica, Bhatia et al., J. Am. Chem. Soc. 114:4432-4433 (1992); and graphite, Delamar, J. Am. Chem. Soc. 114:5883-5884 (1992). This research has been principally directed toward the development of novel composites, Baum et al., Chem. Mater. 3:714-720 (1991); resist materials, MacDonald et al., Chem. Mater. 3:435-442 (1991); biosensors, Pantano et al., J. Am. Chem. Soc. 113:1832-1833 (1991); and biomaterials, Allcock et al., Chem. Mater. 3:450-454 (1991). Recently, surface modification has been combined with photolithography to spatially direct the synthesis of peptides or oligonucleotides, Fodor et al., Science 251:767-773 (1991) and Kiederowski, Angew. Chem. Int. Ed. Eng. 30:822-823 (1991); and immobilization of biopolymers. Rozsnyai et al., Angew. Chem. Int. Ed. Eng. 31:759-761 (1992). Most of the surface modification processes known in the art involve sequential treatment of surfaces with chemical reagents. Id. Only a few such studies have involved the use of azides as surface-modification reagents. Breslow, in Scriven (ed.) Azides and Nitrenes, chapter 10, Academic Press, NY (1984); Harmer, Langmuir 7:2010-2012 (1991).
Examples of existing methods for modifying polymer films include sulfonation of polystyrene, Gibson et al., Macromolecules 13:34 (1980); sulfonation of poly(aryloxy)phosphazenes, Allcock et al., Chem. Mater. 3:1120 (1991); plasma treatment of polyester, Porta et al., Chem. Mater. 3:293 (1991); base hydrolysis of polyimide, Lee et al., Macromolecules 23:2097 (1990); base hydrolysis of polyphosphazenes, Allcock et al., Chem. Mater. 3:1441 (1991); and base treatment of poly(vinylidene fluoride), Dias et al., Macromolecules 17:2529 (1984).
Another conventional method for modifying polymers comprises exposing the surface of a hydrocarbon polymer such as polyethylene with nitrene or carbene intermediates generated in the gas phase. Breslow, in Scriven (ed.), Azides and Nitrenes, chapter 10, Academic Press, NY (1984). Also, difluorocarbene generated in solution has been reported to modify 1,4-polybutadienes. Siddiqui et al., Macromolecules 19:595 (1986).
Perfluorophenyl azides (PFPAs) have been shown to exhibit improved CH-insertion efficiency over their non-fluorinated analogues when the PFPAs were photolyzed in hydrocarbon solvents such as cyclohexane or toluene. Keana et al., Fluorine Chem. 43:151 (1989); Keana et al., J. Org. Chem. 55:3640 (1990); Leyva et al., J. Org. Chem. 54:5938 (1989); and Soundararajan et al., J. Org. Chem. 55:2034 (1990). PFPAs were initially developed as efficient photolabeling reagents. Cai et al., Bioconjugate Chem. 2:38 (1991); Pinney et al., J. Org. Chem. 56:3125 (1991); and Crocker et al., Bioconjugate Chem. 1:419 (1990). Recently, bis-(PFPA)s have been shown to be efficient cross-linking agents for polystyrene, Cai et al., Chem. Mater. 2:631 (1990); and poly(3-octylthiophene), Cai et al., J. Molec. Electron. 2:63 (1991).
Chemical sensors and biosensors are known that exploit certain aspects of surface modification technology. For example, as disclosed in Kepley et al., Anal. Chem. 64:3191-3193 (1992), a mass-sensitive surface-acoustic wave (SAW) substrate is chemically modified by a monolayer of carboxylate-terminated n-alkanethiol molecules terminated by Cu.sup.+2 ions bound to the carboxylate termini. Such a sensor responds reversibly to the nerve-agent simulant diisopropyl methylphosphonate by binding molecules of the simulant to the terminal Cu.sup.2+ ions. Such binding causes a perturbation in the SAW that is proportional to the mass of simulant bound to the terminal Cu.sup.+2 ions.
Chemical and biosensors have also been made that respond to changes in light passing through waveguides. One example of such a sensor is disclosed in Norris, Analyst 114:1359 (1989), wherein the total internal reflectance experienced by light during transmission through an optical waveguide is exploited. Total internal reflection permits the surface of the waveguide to be effectively "interrogated" by the light passing through the waveguide.
In another example, Tan et al., Anal. Chem. 64:2985-2990 (1992), optical fibers are drawn into submicron optical fiber tips that are chemically modified for use as pH probes. Chemical modification of the tips is performed by incorporating fluoresceinamine into an acrylamide-methylene-bis(acrylamide) copolymer and covalently attaching the copolymer to silanized fiber tips by photoinitiated polymerization.